A sucker rod string in a petroleum well extends from a power drive at the wellhead, down through the lengths of tubing and concentric casing used in a production system, and connects to the fluid pump at the lowest elevation in the system. The pump receives an accumulating petroleum inflow from the production zone somewhere above it and lifts a column of fluid up through the tubing. The power drive at the wellhead may be a reciprocating beam system, often called a horsehead, and the sucker rod string then reciprocates a downhole plunger pump which lifts fluid upward through the tubing. The power source may alternatively be a rotary drive coupled via the sucker rod string to a longitudinally extended progressing (or “progressive”) cavity pump.
At the wellhead, the fluid product raised through the tubing is diverted to a lateral outflow and the flow path is closed off by seals about a special interconnection rod which couples the upper end of the sucker rod string to the horsehead or rotary drive. This interconnection rod, called a polish (or polished) rod, must meet stringent requirements because it must provide a low wear durable seal as well as withstand the tensions and stresses engendered by the well operation. It must bear the weight of the sucker rod string as the string cycles, as well as the mass of the fluid column that is being lifted, and it must also resist transitory forces which occur during the pumping, whether reciprocating or rotary. These forces arise variably from impacts, inertial variations and stresses induced longitudinally or by torsion. Since the polish rod reciprocates through fluid seals at the wellhead, it must function uniformly without leaking or undue friction through the operating life of the polish rod. For these reasons, the polish rod diameter is typically larger in diameter than the sucker rods in the string, and is of a suitable high strength material. To provide affective sealing, the surface of the polish rod is spray metal coated with a hard alloy and ground and polished to provide a long life uniform surface which introduces little wear.
Over years of development and use in the oil industry, technical changes have been made to meet constantly increasing artificial lift requirements. Thus a relatively early (pre-1940) specification from the American Petroleum Institute (API) defined a polish rod in which one end had a forged upset area, including a shouldered element having a rod pin machined on it. The other end had no shoulder but terminated in straight threads of a specified API polish rod design. The thread design terminated the straight threads with 3 partial threads along a 9° (nine degree) taper. The shouldered polish rod specification became effectively obsolete while lift requirements were still relatively low because it was felt that there was no structural need for the shoulder. Consequently, both ends of the polish rod have connection threads of the straight API polish rod type without a shoulder.
As artificial lift requirements increased because of increases in the depth of wells along with greater demand for petroleum products, new sucker rod designs were developed for greater performance. It became evident that the polish rod thread configuration often became a weak link in the rod string under more demanding conditions. Shock and impact forces encountered with gas or fluid pounding appeared to be the most frequent failure consideration, because these forces excessively stressed the polish rod connection threads as they alone absorbed longitudinal stresses and impact forces on the rod. Such breakdowns are aggravated because, it is well known, movement between threaded members tends to destroy the connection.
Polish rod connection failures were evidenced, for both beam pumping and progressive cavity pumping, by “belling out” (radial expansion) of the coupling due to the prior sucker rod design. In beam pumping systems the 9° terminal taper and partial threads allowed the tapered threat sections to go deeper and deeper into the coupling, causing threads to shear or even split the coupling. In rotary systems gas absorbed in the stator elastomer, results in torque increasing, which may be accompanied by torque spikes caused by the ingestion, by the pump of formation solids, with failures like those mentioned above.
Applicant has shown that prestressing of connections in sucker rod systems can be used to advantage, as described in Carstensen U.S. Pat. No. 6,942,254 issued Sep. 13, 2005, and Carstensen patent application Ser. No. 09/961,391 filed Jul. 25, 2001. Improvement of the polish rod connection so as to take advantage of these beneficial expedients while providing the needed structural and sealing properties are desirable objectives.